The term "unleavened bread" refers to bread that is made without yeast or other leavening. This kind of bread is flat because it has no leaven to make it rise.
* When God freed the Israelites from slavery in Egypt, he told them to flee Egypt quickly without waiting for their bread to rise. So they ate unleavened bread with their meal. Since then unleavened bread is used in their yearly Passover celebrations to remind them of that time.
* Since leaven sometimes is used as a picture of sin, "unleavened bread" represents the removal of sin from a person's life in order to live in a way that honors God.